The object of the invention relates to a method for producing slush from liquefied gas, by which solid crystals are formed and mixed or are mixed with the liquefied gas to produce slush. The invention also relates to a device for producing slush from liquefied gas.
Hydrogen (H2) is suitable as fuel due to its light molecular weight and its high degree of combustion heat for highly energetic rocket propellant combinations as fuel. Liquid hydrogen (LH2) belongs to the cryogenic fuels and requires that the tanks be appropriately heat-insulated if applied in a motive power unit. Special safety measures must be taken when handling hydrogen. The danger of explosion by hydrogen is greater than in the case of other fuels because of its high diffusion velocity. Even small sparks (e.g. by static charge) is sufficient to cause a fire or an explosion.
Due to the requirements for space transport systems to be constructed to save weight and space, gas liquefaction and further compression of the liquid hydrogen at a low temperature was considered for storage of the driving gas, hydrogen. It was finally proposed to apply slushxe2x80x94with hydrogen crystals mixed with liquid hydrogen, having a crystal content of 40-60% by weight. Slush has a number of favourable characteristics, like high density, high cold content and flows well. Due to these characteristics the slush of liquefied gases is extremely suitable as a cryogenic refrigerating agent not only for space missions but also for terrestrial application.
Methods have already been developed by which hydrogen slush can be produced. The freezing-thawing technology and the Auger methods which have been described in different ways in literature also belong to these methods.
In the freezing-thawing technology, liquid hydrogen which is cooled down to triple point level, is vaporized by creating a vacuum, by which hydrogen crystals are formed on the surface of the liquid hydrogen. The vacuum pressure undergoes cyclic changes to values slightly above and slightly below the triple point pressure, by which a movable matrix of hydrogen crystals forms at a pressure which is below the triple point pressure. Hydrogen condenses on the crystals at a pressure that is slightly above the triple point pressure and the solid particles sink into the liquid. A certain advantage of this method is the relative simplicity of the required technical equipment. However, since the slush production is achieved at underpressure when this method is applied, there is a certain safety risk because an unintentional suction of air, thus the formation of an explosive hydrogen-oxygen mixture should be expected. The production quantity is also limited, perhaps due to the cooling process of the liquid hydrogen which is either achieved by the vaporization of a given quantity of liquid hydrogen via indirect cooling by means of injecting cold helium or by means of a gaseous mixture of helium-neon. The required large quantities of at least some tons per day could not be achieved according to this method up to now because of the commercial expenses involved. To improve the quality of the hydrogen slush also necessitates an aging process of one to two days.
In the case of the second indicated method for producing hydrogen slush, the Auger method, a hollow cylinder filled with helium gas and placed in liquid hydrogen is cooled down to a temperature below the freezing point of the hydrogen. The design enables the formation of solid hydrogen on the inner walls of the cylinder, which is constantly scraped off by means of a swiveling spiral. At the bottom part of the device, the hydrogen crystals which are formed by that method mixes with the liquid hydrogen to slush.
This procedure has the advantage over the freezing-thawing method that the safety risk involved is not given in this case because there is no underpressure. However, it was only possible to apply this method up to now only for the production of quantities for the laboratory. A device which is appropriate for the industrial production of slush according to this method would hardly be able to be realized because of the required mechanical efforts and the costs involved. The slush produced according to this method must also undergo an aging process of one to two days in the long run.
The aim of the invention is now to develop a method for producing slush from liquefied gases, especially for producing hydrogen slush, by which the application of underpressure can be avoided and by which even larger quantities of slush can be produced. Apart from that it should also be possible to improve the quality of the forming slush so that, for instance, the duration of the aging process of the slush can be reduced and the flowing quality of the forming slush in the lines, valves, etc. can be improved. The aim of the invention is also to develop a device which is appropriate for the application of this method, safe to operate and to produce slush according to the industrial standards, with which the aims indicated can also be achieved with as little mechanical effort as possible.
The invention solves these tasks by solid crystals being formed from liquid particles which are released or admitted under pressure to a gas atmosphere, which has a temperature which is below freezing point of the liquid particles.
The method according to the invention thus ensures freezing or crystalizing of the particles in a relatively short space of time. Therefore, the forming solid particles have more of a round shape and there is a good quality of slush at increasing density. Due to the round shape either no or a slight aging process is necessary in order to ensure a high density of the slush and good flowing. No underpressure is required for producing the slush so that safety risk is reduced extremely in case hydrogen slush is produced.
According to another feature of the invention, the liquid particles are formed in a special and simple way by atomization of the liquefied gas.
For this process only an appropriate atomizing device device is required which can be mounted in a simple manner and consists of at least a nozzle or a centrifugal and mixing chamber or similar.
The atomization process can be effected in a simple manner at least partly by gas being supplied to the atomizing device under pressure which preferably is according to the gaseous phase of the supplied liquefied gas.
Therefore the liquefied gas supplied to the atomizing device can be cooled down before and/or during the atomizing process by means of a gaseous cooling medium. Additional cooling-down shortens the period of crystalization.
The method can be designed in such a way that either additionally or alternatively to atomization and by applying a supplied gas which is pressurized, atomization can be effected at least partly when the liquefied gas discharges into the cold gas atmosphere.
In order to ensure atomization which is as optimum as possible, i.e. bursting of the liquid jet discharging from the atomizing device, the pressure in the cold gas atmosphere is set at a rate which is below the corresponding critical pressure at the discharge aperture of the atomizing device.
In order to minimize the safety risk, the pressure in the cold gas atmosphere where crystalization occurs is set in such a way so that it corresponds at least to the ambient air pressure, it will particularly be at a value that is slightly above the ambient air pressure.
The inventive method has the special advantage that the forming slush can be continuously drained off essentially during the production of new slush. For this process it is favourable if draining-off of the slush is controlled by constant measurement of its density, which ensures the same quality of the drained slush.
The inventive method is especially suitable for producing hydrogen slush which has become more important as cryogenic fuel.
In this case helium gas is suitable as a cooling medium which is to be supplied to the atomizing device.
There is an advantage when a helium gas atmosphere, which can be made available at a corresponding low temperature, is applied as a cold gas atmosphere where the liquid particles freeze or crystalize.
The present invention also relates to a device for producing slush from liquefied gas using a cryostat container, which is partly filled with liquefied gas that mixes with solid crystals to produce slush. The inventive device is characterized by an atomizing device arranged in the cryostat container for forming liquid particles from supplied liquefied gas. This device discharges the liquid particles above the gas in the container into a gas atmosphere, which has a temperature below the freezing point of the liquid particles.
The inventive device thus ensures in a simple manner that crystals can form in a relatively short space of time, which ensures a high quality of slush.
The atomizing device can be mounted simply and consists of at least a nozzle or a centrifugal and mixing chamber or similar.
In order to ensure atomization of the supplied liquefied gas, different possibilities are available. There is one design that is especially simple by which the atomizing device has a supply line for the pressurized gas which thus effects atomization within the atomizing device.
In another design, the discharge aperture of the nozzle of the atomizing device is designed in such a way so that the set pressure there is higher than that in the cold gas atmosphere outside of the pressure of the atomizing device. In this type of design, the jet can burst when discharging out of the atomizing device thus causing an optimum turbulence of the forming liquid particles.
In order to maintain the crystalization period as short as possible, the atomizing device can have another supply line for a gaseous cooling medium. In case of a design with a centrifugal or mixing chamber, the supply of a cooling medium in the atomizing area is especially profitable.
In a preferable design of the atomizing device, the gaseous cooling medium is supplied in a pressurized state by means of gas nozzles and then led into the atomizing device in a gas guide cone which forms a discharge gap in the area of the nozzle. In this design the gas jet is not only a cooling medium, it also contributes to a turbulence of the liquid particles and the forming crystal particles discharging from the nozzle.
In order to ensure as far as possible that there is a constant cold gas atmosphere in the cryostat container, it is of an advantage, if the container has a number of inlets for supplying the gas which forms the cold gas atmosphere. These inlets shall be specially arranged as a sprinkler. This also enables a quick desired change of pressure.
A continuous production of slush is possible with the invention because it can be provided with devices to drain off the forming slush and to supplement the quantity of liquefied gas during the manufacturing process. In order to ensure the good quality of the drained slush, the device is equipped with a system for measuring the density of the slush which controls draining of the slush.
The device can be profitably applied to produce hydrogen slush from hydrogen.
In this case the pressurized gas supplied for atomization within the atomizing device will be hydrogen which will be also an advantage.
Helium gas is especially suitable in this case as cooling medium for the supplied liquid hydrogen.
Helium is also especially suitable for the cold gas atmosphere within the cryostat container because it ensures the required low temperatures.
The invention can be developed into a production plant directly, with which the larger quantity of slush can be produced. It can be equipped, for example, with a number of atomizing devices which can be connected as a ring or similar.